A solid diffusion source comprising glass ceramic or polycrystalline particles is disclosed in the Rapp U.S. Pat. No. 4,141,738. In U.S. Pat. No. 4,033,790 there is disclosed solid diffusion sources made by hot pressing R.sub.2 O.sub.3 /P.sub.2 O.sub.5 (La.sub.2 O.sub.3 /P.sub.2 O.sub.5, Ce.sub.2 O.sub.3 /P.sub.2 O.sub.5 and Y.sub.2 O.sub.3 /P.sub.2 O.sub.5) compounds in which the R.sub.2 O.sub.3 to P.sub.2 O.sub.5 is mainly 1 to 5. The above U.S. patents are incorporated by reference.
U.S. Pat. No. 4,846,902 discloses a solid diffusion source that is a Gd oxide/P.sub.2 O.sub.5 compound in which the ratio of Gd oxide to P.sub.2 O.sub.5 to about 1/1 to 1/5.
It is desirable to provide a solid diffusion source that is easily and efficiently made, and a source that evolves P.sub.2 O.sub.5 relatively rapidly at a relatively high temperature.